The purpose of this study is to determine how the performance on various reaction time and thinking tasks, sleepiness and mood, brain wave activity, body temperature, blood hormone levels are affected by limiting the nightly sleep. Sleep loss is common in modern society, and we want to test theories about how adult men and women respond to extended periods of curtailed sleep. We want to gather information on sleep, waking brain waves, body temperature, blood hormones, and performance patterns during 13 24-hour periods (13 days) in the GCRC at the Hospital of the University of Pennsylvania or the Clinical Neuro-Diagnostic Unit at the Hospital of the University of Pennsylvania where total sleep hours are controlled.